This invention relates to an angle modulated wave demodulation system and, more particularly, to a system for demodulating an angle modulated wave signal picked up from a multi-channel record disc.
The applicants of the present invention have heretofore proposed a recording and reproducing system for a four channel record disc (multi-channel record disc) in the U.S. patent application Ser. No. 92803 filed on Nov. 25, 1970 and now patented under No. 3686471, entitled "SYSTEM FOR RECORDING AND/OR REPRODUCING FOUR CHANNEL SIGNALS ON A RECORD DISC". By this proposed system, in the recording system, sum and difference signals are formed respectively from signals of every two channels of the signals of four channels. More specifically, from signals respectively designated by the notations Ch1, Ch2, Ch3, and Ch4 representing the first through fourth channels, sum signals (Ch1 + Ch2) and (Ch3 + Ch4) and difference signals (Ch1 - Ch2) and (Ch3 - Ch4) are formed. Thereafter, the difference signals are angle modulated, and the angle-modulated wave difference signals F(Ch1 - Ch2) and F(Ch3 - Ch4) are of a band which is higher than the above mentioned direct-wave sum signals. These signals are mixed with the direct-wave sum signals (Ch1 + Ch2) and (Ch3 + Ch4).
The two multiplexed signals {(Ch1 + Ch2) + F(Ch1 - Ch2)} and {(Ch3 + Ch4) + F(Ch3 - Ch4)} of the direct-wave sum signals and the angle-modulated wave difference signals are recorded by cutting them on the left and right walls, respectively of a groove of the 45 - 45 system on a record disc.
In the reproducing system, the reproduced multiplexing signals are respectively separated into direct-wave sum signals and angle-modulated wave difference signals. The latter signals are demodulated, and the original difference signals are again obtained. The sum signal (Ch1 + Ch2), difference signal (Ch1 - Ch2), sum signal (Ch3 + Ch4), and difference signal (Ch3 - Ch4) obtained in this manner are respectively matrixed. Thus the original signals Ch1, Ch2, Ch3, and Ch4 of the four individual channels are again obtained. These signals are reproduced audibly from four loudspeakers disposed respectively at left front, left rear, right front, and right rear positions relative to a listener.
In some cases, an absence of a signal or an abrupt decrease in the signal level occurs in a signal picked up by a pick-up stylus. This occurs, for example, when a pick-up stylus rides over a particle of dust in a groove of a record disc and fails to forthfully and completely trace along the walls of the groove. Also, when a part of the groove is damaged, a part of the wave form on the groove is lacking. Sometimes, the waveform on the groove is so fine and complicated that the pick-up stylus jumps over it and fails to completely trace the waveform. Sometimes, a part of the groove has been worn away due to repeated reproductions of the record disc by using a pick-up cartridge having a stylus tip of a large equivalent mass, i.e. a large moment of inertia.
As stated above, the frequency range of the angle modulated difference signal (20KHz to 40KHz) recorded in the signal groove is higher than the range of the direct wave sum signal. Accordingly, a part of the waveform having a relatively gradual curve relates to the direct wave sum signal and a part having a relatively small and fine waveform relates to the angle modulated wave difference signal. Therefore, the absence of signal and the drop in the level in the reproduced signal in each of the above described cases occurs particularly in the angle modulated wave difference signal.
In the portion of the recording where the absence of signal and the drop in the level occurs, the frequency of the carrier component in the angle modulated wave difference signal has abruptly deviated to substantially a very low frequency. Accordingly, noise is generated in an angle demodulator in a later stage in the portion where the carrier wave has thus abruptly deviated to a low frequency.
The abrupt decrease in the angle modulated wave and the resulting generation of noise is characteristic of a reproduction of the multi-channel record disc and is essentially different from a noise generation which generally occurs in FM communication etc. In FM communication, the carrier is always maintained at a constant level. If a noise signal is added thereto, a result is that the level as a whole increases rather than decreases. Furthermore, a change in the level of an angle modulated wave observed in a fading phenomenon is gradual and not abrupt. Whereas in the multi-channel record disc, it is inevitable that the level of the angle modulated wave decreases abruptly due to the relationship between the tip of the stylus and the groove, existence of dust, damage of the groove and other causes.
In order to prevent the reproduction of the abnormal noise as described above, the applicant previously proposed a muting system in which the absence of the signal in the angle modulated wave is detected and the difference signal system is thereby switched OFF. According to that proposed system, however, the muting circuit frequently switches between ON and OFF as the disc is reproduced if, for example, the sound groove is half damaged. In this case, the reproduced sound field is frequently switched between the two channels and four channels so that orientation of the sound field changes thereby producing a undesirable accoustic effect.
On the other hand, a method may be conceivable in which the sound volume of the difference signal is continuously changed according to the level of the angle modulated wave, depending upon damage of the sound groove. This method, however, is disadvantageous since the separation of the signals is poor because the level of the sum signal does not change in accordance with the damage of the sound groove. Alternatively, a system is conceivable in which the frequency characteristic of the difference signal is changed in accordance with the change in the level of the angle modulated wave. This system is also disadvantageous in that the phase characteristic in the middle frequency band changes although the frequency characteristic in a middle frequency band does not change in response to the change in frequency characteristic in high frequency band. As a result, the separation characteristic of the middle frequency band, as well as of high frequency band, is deteriorated. Therefore, none of the above described methods are effective to eliminate the noise accompanying the absence of signal and the abrupt drop in the level in the angle modulated wave, and none are suitable for practical use.